The invention relates generally to level gauging, and more specifically to a bottom reflector for a radar-based level gauge.
Radar-based methods are since several years extensively used for level gauging in various tanks. In such level gauging a distance from the top of a tank to a surface of a liquid stored in the tank is measured by means of transmitting microwaves towards the surface of the liquid, receiving the microwaves as reflected against the surface of the liquid, and calculating the level of the liquid in the tank from the propagation time of the transmitted and reflected microwaves.
An intrinsic limitation of radar-based level gauging is that many liquids are at least to some extent transparent to the microwaves used, which may possibly cause microwaves reflected from the bottom of the tank to interfere with the microwaves reflected from the surface of the liquid when measuring low liquid levels, i.e. the distance resolution may be insufficient to distinguish the microwaves reflected from the bottom of the tank from those reflected from the surface of the liquid. This, microwaves reflected from the bottom of the tank are likely to disturb the measurement at low liquid levels. Depending on the type of radar level gauge this problem may arise at levels below a few tenths of a meter. This is a particularly common problem in tanks having a flat bottom, which comprise liquids like petroleum products, which are at least slightly transparent.
In some tanks the bottom may be inclined. In such tanks there is typically no problems of interfering radar echoes, but on the other hand no microwaves reflected from the bottom of the tank are received when the tank is empty as the radar beam is deflected away from the receiver of the level gauge. If the tank is empty it is desirable to have a reasonably strong radar echo from the bottom in order to establish such condition. In other cases there may be an unknown layer of sludge on the bottom of the tank causing even a flat bottom to give an ambiguous radar echo or no radar echo at all.
The problem of not receiving bottom radar echoes in empty tanks may be solved by having a simple reflecting structure welded on the bottom of the tank. However, such a reflecting structure will cause interferences at liquid levels slightly above the reflecting structure provided that the liquid in the tank is at least partially transparent to the microwaves.
A main object of the invention is thus to provide a bottom reflector for a radar-based level gauge aimed for measurements of the level of a liquid in a tank, which bottom reflector reduces the interference from microwaves reflected from the bottom of the tank when using the gauge for level measurement at low liquid levels, but still provides for a satisfactory refection of the microwaves when the tank is empty.
In this respect there is a particular object of the invention to provide such a bottom reflector, which is suitable to be used whether the level is gauged through a pipe or not.
A further object of the invention is to provide such a bottom reflector, which is suitable to be used irrespective of whether the radar-based level gauge may be a frequency modulated continuous wave (FMCW) radar apparatus, a pulsed radar apparatus, or any other type of distance measuring radar.
A still further object of the invention is to provide such bottom reflector, which is simple, reliable, efficient, accurate, precise, easy to manufacture and install, and of low cost.
These objects, among others, are attained by bottom reflectors as claimed in the appended patent claims.
The inventor has found that by providing a bottom reflector, which is mountable at a predetermined height in the tank, preferably close to the bottom of the tank, for reflecting the microwave signal when no or only very levels of liquid is present in the tank, and comprises a reflecting structure, which has a first reflection coefficient for the microwave signal when the level of the liquid is above the reflecting structure and a second reflection coefficient for the microwave signal when the level of the liquid is below the reflecting structure, wherein the first reflection coefficient is substantially lower than the second reflection coefficient, the above objects can be attained.
Preferably, the liquid is an oil product such as e.g. crude oil, a liquid petroleum gas (LPG), a liquid natural gas (LNG), other liquid hydrocarbons, or liquids in general which are at least partly transparent to microwaves. Such products have typically a dielectric constant in the range 1.6-3, whereas the atmosphere above the level has typically a dielectric constant in the range of 1-1.03 depending on the contents and pressure thereof.
Still preferably, when the level of the liquid is above the reflecting structure the microwave signal reflected from the reflecting structure is weaker, advantageously much weaker, than the microwave signal reflected from the level of the liquid. When the level of the liquid is below the reflecting structure the microwave signal reflected from the reflecting structure is preferably comparable to, or slightly stronger than, the microwave signal reflected from the level of the liquid.
Various reflecting structures can be used for obtaining the desired functionality based on the differences in dielectric constant of the liquid and the atmosphere there above. These reflecting structures are described in detail in the present description and can be grouped into different groups of preferred embodiments: cut-off grids, resonant structures such as dipoles, dielectric reflectors, polarization turning structures, microwave guiding structures bendable over a small distance for guiding the transmitted and reflected microwave signal, and structures particularly applicable when free space propagation of the microwaves is used.
With respect to the group of polarization turning structures it shall be appreciated that the microwave signal as used in the level gauging has a specified polarization state and that the first and second reflection coefficients are given for that specified polarization state. Thus, the effect of rotating the polarization of a linearly polarized microwave signal at reflection in the liquid is equivalent to decreasing the reflection coefficient for the microwave signal in its specified polarization state.
A main advantage of the present invention is that level gauging can be performed without any interference from microwaves reflected from the bottom of the tank. When the tank is empty, or nearly empty, i.e. when the reflecting structure of the bottom reflector is raised above the level of the liquid gauged, a distinct reflex from the bottom reflector is obtained, which is an indication of the empty tank or the low liquid level.
The radar-based level gauges are used to measure levels in tanks, which for the purpose of the present invention include not only large containers but also processing apparatuses such as, for example, reactors, centrifuges, mixers, hoppers, graders, or heat-treatment furnaces and similar devices, which are used in food chemistry, pharmaceutical chemistry, biochemistry, gene chemistry and petrochemistry.
Further characteristics of the invention, and advantages thereof, will be evident from the detailed description of preferred embodiments of the present invention given hereinafter and the accompanying FIGS. 1-7, which are given by way of illustrated only, and thus are not limitative of the present invention.